Charge polarity-dependent ion-insertion asymmetry during electrochemical doping of an ambipolar π-conjugated polymer

Electrochemical doping is central to a host of important applications such as bio-sensing, neuromorphic computing and charge storage. However, the mechanisms that enable electrochemical dopability and the various parameters that control doping efficiencies are poorly understood. Here, employing complementary electrochemical and spectroelectrochemical measurements, we report a charge-polarity dependent ion insertion asymmetry in a diketopyrrolopyrrole-based ambipolar π-conjugated polymer. We argue that electrostatic interactions are insufficient to fully account for the observed charge-specific ion insertion into the polymer matrix. Using polymer side-chain dependent electrochemical doping studies, we show that electron density donating and accepting tendencies of polymer side-chains sufficiently describe the observed charge-polarity dependent electrochemical doping. Our observations are akin to the solvation of dopant ions by polymer side-chains. We propose that Gutmann donor/acceptor number framework qualifies the ‘solvent-like’ properties of polymer side-chains and provides a rational basis for designing π-conjugated polymers with favorable mixed ionic electronic transport properties.


Determination of HOMO/LUMO levels of DPP-based polymers
The Ag + /Ag reference electrode potential was calibrated in a 0.1 M TBAPF6 in acetonitrile containing a small amount of ferrocene. The E1/2 for ferrocenium/ferrocene redox, EAg+/Ag (Fc + /Fc) was determined to be 0. 16    Ag/AgCl with 1M NaCl fill solution) occur in the backward direction resulting in O2 evolution: The redox reaction involved in p-type doping is given by equation 3.
Since the reaction, 2 + -+ 2 ( + 2 . ⇌ 2 ( + --has Eredox = 0.033 -0.58 = -0.55 V < 0 , it is not feasible thermodynamically. In other words, the deep HOMO level of 2DPP-OD-TEG at -5.25 eV implies H2O2 generation by ORR in ambient conditions is suppressed (see Supplementary Fig. 6). 3 As a result, the reaction involving O2 evolution from H2O2 can be discounted. The relevant parasitic reaction during p-type electrochemical doping of 2DPP-OD- polymeric surfaces are known to be high. 2 Therefore, the fraction of charge that leads to the acidification of the electrolyte is expected to be small.
The parasitic water decomposition/oxygen evolution reaction can cause de-doping of the channel through the reaction given by equation 4 (also see Supplementary Fig. 6).
To ensure that this is not the reason for lack of p-type doping with smaller anions such as F -

Relation between ion transfer free energy and Gutmann DN/AN
The net n-type and p-type electrochemical doping reaction is represented by equations 1 and 2 respectively Electrochemical doping process can be thought of as composed of two processes namely 1) ion insertion from the electrolyte into the polymer and, 2) polymer redox. The ion insertion process is analogous to an ion transfer process between two solvents and involves a free energy of with the solvent DN using equation 5 where the coefficients $ ! and $ ! are related to $ ! ? and ′ $ ! through the Faraday constant, and the reference electrode potential; particularly, $ ! = $ ! ? .
The solvation energies of the ion can therefore be rewritten in terms of the solvent and polymer DN as equation 6 and 7.
Using equation 3, the free energy of transfer of the ion ( from the solvent to the polymer is then proportional to the difference in their donor numbers. We can write down the free energy of transfer as: Gritzner calculated the values of ′ $ ! and ′ $ ! for various cations and the values are tabulated below 9 Cation > 0 leading to ion insertion process incurring an energy penalty. The larger the difference in DN, the larger the thermodynamic energy penalty and the more difficult it is to insert the ion. Further, $ ! decreases with increasing cation size implying ion insertion energetics are more sensitive to the difference in DN for smaller cations.
Similarly, since anion coordination depends on Lewis acidity/electron density accepting properties of the solvent, the free energy of solvation 10 and therefore the free energy of transfer of anions between two solvents would depend on the difference in AN of the two solvents implying ∆ D # ,412→0123 A6=B4C76 for anions can be written as: Here D # is an ion dependent parameter which represents the slope of the ∆ D/D # ,412 67;19 vs.

Passive swelling of 2DPP-OD-TEG in aqueous electrolyte
Passive swelling of 2DPP-OD-TEG was assessed using atomic force microscopy (AFM). The polymer film were prepared for swelling studies by spin coating on ITO/glass. A thin scratch was made on the film using the point of a hypodermic needle. The film was first imaged across the scratch, in the dry state. A sample AFM image of the dry film is shown in Supplementary   Fig. 19a. Next, the film was covered with aqueous 0.1 M NaCl solution and allowed to soak for 1 hour. Then AFM measurements were performed on the film (Supplementary Fig. 19b) while still in contact with the electrolyte using in-liquid imaging mode, to prevent any drying/shrinking of the film. Multiple measurements were performed and mean terrace heights were calculated using Gwyddion AFM analysis tool ( Supplementary Fig. 19c). 11 Supplementary Figure 19 N-methyl formamide 27 32.1 [8] N,N-Dimethylformamide 26.6 16 [17,8] N,N-Diethylformamide 30.9 17.8 [17,14] N,N-Dimethylacetamide 27.8 13.6 [17,8] N,N-Diethylacetamide 32.2 13.6 [17,14] Phosphor- Tributyl phosphate 23.7 9.9 [14]